The following disclosure relates to electrical circuits and signal processing.
A phase interpolator is a circuit that can be used within an integrated circuit to adjust a phase of a signal (e.g., a clock signal). A phase interpolator can be used in a variety of applications. For example, a phase interpolator can be implemented within a receiver to adjust a phase of a sampling clock, or within a write precompensation circuit (e.g., of a hard disk drive system) to adjust a phase of a write clock.
FIG. 1 shows one example of a conventional phase interpolator 100. Phase interpolator 100 includes a differential pair having a pair of differential inputs PH1, PH1Bar (complement of PH1), PH2, PH2Bar (complement of PH2), and differential outputs OUT1, OUT1Bar (complement of OUT1). Phase interpolator 100 includes bias currents I1-I2, transistors M1-M4, and resistors R1-R2. In general, phase interpolator 100 provides a phase shift for a signal that is an interpolation between phase signals PH1 and PH2. Phase interpolator 100 provides the phase shift based on bias currents I1 and I2. For example, if bias current I1 is turned off, then phase interpolator 100 provides an output signal OUT1 having a phase that is substantially equal to that of phase signal PH2. And if bias currents I1 and I2 are substantially equal, then phase interpolator 100 provides an output signal OUT1 having a phase that is substantially in between those of phase signals PH1 and PH2.
As bias current I1 or I2 is changed, a common mode component of output signals OUT1 and OUT1Bar varies, and phase interpolator 100 therefore requires a certain amount of time to settle in order to provide an output signal having an accurate phase shift. In addition, phase interpolator 100 may require a large buffer to drive each of phase signals PH1, PH1Bar, PH2 and PH2Bar. Such buffers typically require a large power supply (e.g., greater than 1.5 Volts). A change in a bias condition (e.g., including a large power supply) can impact interpolator linearity.